The present invention relates to a distance measuring method that uses a standing electromagnetic wave produced by interference between a traveling electromagnetic wave and a reflected electromagnetic wave irrespective of their frequencies to thereby measure a distance between a distance measurement system and a measurement object, and further relates to a distance measuring device and a distance measuring structure each using such a method.
As distance measuring devices using radio waves, there have been known radio wave radars using microwaves or milliwaves. These radio wave radars are classified into pulse radars, FMCW radars, and so forth depending on their modes, and recently, spread spectrum radars and CDMA radars have also been available.
Specifically, the pulse radar transmits a pulse signal toward a measurement object and measures a time from a time instant of the transmission of the pulse signal to a time instant when the pulse signal has returned after reflection from the measurement object, to thereby derive a distance from the radar to the measurement object. The FMCW radar transmits a frequency-swept continuous wave toward a measurement object and derives a distance from the radar to the measurement object based on a frequency difference between a transmission signal and a reflected signal. In this case, the moving speed of the measurement object can also be measured simultaneously. The spread spectrum radar and the CDMA radar are basically the same as the pulse radar and each measure the distance based on a propagation time going to and from the measurement object.
In case of those radio wave radars, however, since the minimum detectable distance is several tens of meters or more, there is a problem that the measurement becomes difficult if the measurement object is located at a short distance. On the other hand, in case of a Doppler radar known as a radar other than the foregoing radars, there is a problem that although it is simple in structure and enables the measurement even if the measurement object is located at a short distance, the distance to the measurement object can not be measured if the measurement object is stopped. Further, in case of those conventional radars, there is a problem that when a plurality of radars are simultaneously used nearby, since each of their receivers has no means to avoid reception of signals transmitted from the other radars, the measurement error increases or the measurement is disabled.
In view of this, JP-A-2002-357656 (Literature 1) has proposed a technique that enables measurement of a distance to a measurement object with high accuracy even if the measurement object is located at a short distance. The technique of Literature 1 is based on an idea that when an electromagnetic wave is transmitted from an electromagnetic wave generating source toward a measurement object as a traveling wave, if there occurs a reflected wave from the measurement object, a standing wave is produced irrespective of their frequencies. Specifically, this technique calculates a period of amplitude of the standing wave at a detection point that is offset by a predetermined distance from the electromagnetic wave generating source toward the measurement object and derives a distance between the detection point and the measurement object based on the calculated period.
The technique of Literature 1 is effective when the measurement object and a distance measurement system are both stopped or moving at the same speed (when the relative speed therebetween is zero). However, there is a difficulty that when the relative speed is not zero, the measurement error increases to disable accurate measurement of the distance to the measurement object.
As other techniques relating to the distance measurement, JP-A-H05-203412 (Literature 2) discloses a device that measures a position where the refractive index of light changes, JP-A-S38-1257 (Literature 3) or JP-A-S58-198781 (Literature 4) discloses a distance measuring device using a light beam, and JP-A-H06-10082 (Literature 5) discloses an optical distance measuring device. The techniques of Literatures 2 to 5, however, do not even use the standing wave and therefore can not accurately measure the distance to the measurement object.
Further, JP-A-S59-142485 (Literature 6) discloses a distance measuring method, and JP-A-H05-281341 (Literature 7) discloses a method and device for distance measurement. The technique of Literature 6 changes a frequency to measure a resonant frequency and derives a distance from the resonant frequency. The technique of Literature 7 relates to measurement of a distance based on a period of a standing wave. However, either Literature 6 or Literature 7 does not positively utilize the standing wave and therefore can not achieve a measurement accuracy that is satisfactory.
Further, JP-A-2002-296344 (Literature 8) or JP-A-H01-219583 (Literature 9) discloses a distance measuring device, and JP-A-H02-304387 (Literature 10) discloses a distance measuring method using interference of electromagnetic waves.
The technique of Literature 8 carries out distance measurement with respect to a stationary object by the use of a Doppler sensor. Specifically, the distance is measured using a waveform of a standing wave between the sensor and the measurement object. However, when the relative speed therebetween is not zero, the measurement error increases so that a satisfactory measurement accuracy can not be achieved.
The technique of Literature 9 aims to perform frequency modulation relative to a laser diode with high accuracy in the optical heterodyne interference method that obtains information from optical beats, and employs a counter for counting signals corresponding to a beat component. In this technique, use is made of a bandpass filter for extracting only those frequency components around the beat component and, by changing the frequency with a modulating signal such as a sawtooth wave to measure a level corresponding to an optical beat signal caused by a transmission signal and a reception signal, the distance is measured. To this end, the measurement is difficult when the measurement object is located at a short distance.
The technique of Literature 10 enables measurement of a short distance using interference between a transmitted electromagnetic wave and a reflected wave from an obstacle (measurement object). Specifically, a reflected wave obtained by transmission of an electromagnetic wave, hitting thereof on the obstacle, and reflection thereof from the obstacle is made an object to be measured. Since the transmitted electromagnetic wave and the reflected wave are individually utilized, it basically requires a considerable time to carry out distance measurement once although depending on a length of time for transmission of the electromagnetic wave and the number of cycles thereof.